This invention relates to drive assemblies comprising a plurality of friction disks and reaction disks arranged along a common axis and which can be brought into and out of axial engagement to perform a clutch or braking function. These assemblies are usually installed in a housing having a linear actuator at one end of the assembly and a axially fixed wall or surface at the other end. The assemblies are commonly used in automobiles, trucks and other land vehicles, as well as stationary power transmissions.
One known type of friction assembly comprises a plurality of circular friction disks having a central opening and being slidably mounted on a central shaft or other central element, together with bare metallic reaction disks interleaved with the friction disks. Alternating disks have respective inner and outer splines engaged with a central element such as a shaft and an outer element. Each friction disk comprises a relatively rigid core plate made of steel and having an annular ring of relatively thermally nonconductive friction material bonded to both sides of the core plate. In a clutch, the alternating disks move at different rotational speeds while disengaged, and move at the same speed upon engagement. In a brake, one of the alternating disks is fixed with respect to the housing in which the brake is mounted. The other disks rotate when the brake is disengaged and do not rotate when they are engaged. These assemblies typically operate in a liquid lubricant to provide cooling.
In the above type of conventional assembly, alternating disks carry a ring or annular band of friction material on both sides, and bare metal reaction disks are interleaved between the friction disks, with the bare metal surface on each side of the reaction disk being brought into sliding engagement with a ring of friction material from a friction disk.
Another type of assembly is known as a single sided assembly in which each disk carries a ring of friction material on one side only facing in the same axial direction. Upon engagement, the friction material ring of one disk engages the bare metal surface of an adjacent disk, which provides a heat sink for absorption of heat.
In a double sided design, heat does not penetrate to the steel core of the friction disk since it is protected on both sides by low conductive friction material. Therefore, the core plates of the friction disks do not contribute much to heat sink, and the only contributors are the bare metal reaction disks. By contrast, in the single sided design, each piece of metal inside the pack is exposed to heating and can be considered as being more effective than in the double sided design. The advantages of a single sided assembly are that such assemblies allow a reduction of overall thickness of the pack of disks or an increase in capacity of the same overall thickness, as well as a reduction of temperature for the same overall thickness.
The most critical stage of clutch and brake operation is the engagement phase which starts when the alternating disks move at different rotational speeds. During this phase the actuator force is being applied to the disk assembly in order to produce frictional torque and thereby reduce the initial speed difference. Both the sliding and the frictional torque occur simultaneously during this stage and it results in generation of significant amount of frictional heat. The disks are being exposed to high thermal loads which substantially contribute to material wear and which are the major cause of failures of these parts.
Double sided assemblies are often considered as being more robust than single sided assemblies. Thus, there is a continuing need to improve the performance of single sided assemblies in view of the benefits associated therewith. A reduction in overall thickness in tightly packed transmissions is often considered as an important and valuable advantage.
In accordance with the present invention, a single sided friction disk assembly is provided, and a modification is made at one or both ends of the assembly to cause more uniform contact between the first and second disks and the remaining disks in the series, thereby resulting in more uniform heating and reducing thermal stress and thermoelastic deformations. These modifications help to avoid a concentration of the contact pressure at the mean radius of the first and relatively cool disk, whose bare metal surface is not engaged by another disk, and the second disk in the assembly, which is engaged by the first disk, as will be described in more detail herein. The end disks are supported in a manner to accommodate or allow normal thermoelastic deformation, with essentially no support being provided in a zone of mean radius between the inner and outer circumference of the first disk.
In one preferred embodiment, the actuator or part which engages the first disk is provided with an annular groove which is smaller in width that the ring of friction material on the other side of the disk. The same effect can be achieved by applying a pair of radially spaced support rings on the rear or metal side of the first disk. This allows the first disk to assume the thermally deformed shape of the second disk.
In addition, a modification may be made at the other end of the assembly, or the end which faces the friction rings on the disks. In this embodiment, a support ring is provided on the axially fixed wall or the axially moved end reaction plate of the assembly. This support ring is smaller in width than the width of the annular friction rings on each disk, so that the entire assembly is allowed to assume a concave shape relative to the support.
As used herein, the term xe2x80x9cwidthxe2x80x9d refers to the radial dimension between the outer and inner circumferences of the ring of friction material, and the reduction in width means a reduction in outer diameter and/or increase in the inner diameter. These changes are normally equal in magnitude but may differ somewhat to compensate for a particular assembly.
In summary, the present invention is directed broadly to single sided friction disk assemblies in which interleaved bare metal reaction plates are eliminated and an annular ring of friction material on one disk engages the rear metal surface of an adjacent disk. The axially movable disks are mounted between a pair of end supports, one of which is axially fixed and the other being axially movable to compress and release the disk assembly. Upon engagement, at least some of the disks thermally deform, assuming a concave shape at the mean diameter on the friction material side between the inner and outer diameter of the annular friction ring. Means are provided at or near one and preferably both end supports to allow this thermal deformation to occur without interference from the end supports.